Hello Everyone.

Vehicle 2002 Buick Century 3.1L engine

Problem: Intermittent variation in Idle Speed.

Bought the car with the following conditions.

a- Horn must be in short. Fuse was removed by previous owner. If I put fuse horn activates.

b- Instrument Cluster Panel non operating. Can't see gear selection indicator or odometer.

c- High beam only no switching to low beam.

Have taken car to couple mechanics and discussed the matter with a few " on line experts". Has eluded everyone so far.

NO DTC'S.

Vehicle most of the time starts the "warm up" procedure and RPM comes down into " normal " range.

After a while ( car has not been driven, just idling on driveway) RPM starts to increase. most of the time gets up to 2500 RPM range.Sometimes it comes back and then goes back up again.

The worst I had was in a day when I drove it for about 40 minutes and the RPM was around 3400 RPM. I got off the highway and I couldn't get the RPM to come down. I disconnected the TPS and IAC sensors but engine just stayed at 3400 RPM ( read on Autel code reader)

Another time drove it for about 1 hour everything was noraml.

The following was replaced IAC sensor ( tip looked dirty), TPS, PCV valve tube with U hose ( U hose was collapsing when engine was running).

It appears that  something needed to be repaired ( work done by someone else before me) on the low reference side ( orange black wire) of ECT and MAP sensor.A  new replacement wire has been installed. The replacement wire runs from MAP ( terminal A  circuit 469) to splice 121 then to ECT ( terminal A) and also to PCM Connector C1 Part 1, terminal 13 ( orange/blk) low reference circuit 469). This wire goes through connector 102 terminal L.

I have checked Connector 102 terminal K for 5V reference. It is OK.

I was suggested to buy a TECHII scanner, unknowing I purchased a genuine "FAKE" TECH II, it appears to work for my purposes and based on what I see on that scanner I have noticed the following.

The TPS voltage and %, as per reading on scanner, keeps on increasing after the vehicle been running for a while, without any action or intervention on my part. The throttle on this vehicle is mechanical and nothing has been actuated to create the changes that are displayed on the scanner. MAF g/s and IAC count simultaneously increase and also RPM. All 3 parameters increase at the same time.

QUESTION THAT NO ONE HAS ANSWERED SO FAR. What can be the cause of the raising voltage on the TPS and the IAC count? Who is driving who? TPS drives IAC or IAC drives TPS, or is there something else that drives both?

I did notice that someone did something to one of the connectors at the PCM because the Plastic cover for one of the connectors has some broken plastic tabs. Opened up everything seemed OK.

I found out that there is a GM tool that can actually control the IAC count. ( J37027-1A) . With this tool the IAC sensor is disconnected and the RPM is actually be controlled, even to somewhat below the " normal" ( +-700 RPM) range.

I have the RPM a little lower and after the vehicle warms up, it can be driven, sometimes a little hesitation is felt if a harder acceleration is tried. In normal driving speeds it seems ok.

The last act on this is that I plugged the IAC and the idle RPM remained normal for a while.

As as it goes with old cars, I haven't driven after that, because as I went to raise the RF window , regulator went out, and as I was backing up another vehicle to bring the Buick in, I thought I heard some " strange " noise, but it was muffled by the diesel engine on the vehicle that I was backing up.

When I got to the Buick I couldn't hear anything out of the ordinary, but upon opening the hood i noticed something wet on the radiator frame, it felt slightly oily and slippery- hello Dexcool - I could also see the fine line on the hood liner and firewall,( liquid thrown by a rotating pulley) Water pump, anyone? YES the pump just went and off course the chocolate mousse is all over ( Dexcool + Regular antifreeze- never fails)

Looking at the bright side ( What????) I now know learned about the MAF, ECT. IAT, TPS, MAP, PCV, DTC. MIL, CAN, PCM, ECU,LTFT,EVAP, VSS, OPEN LOOP, CLOSED LOOP,TCM, EGR, O2S,WOT, 5 V Ref, LOW Ref, Pull UP and Pull down resistor, Voltage drop, Open, short to ground, short to power, Ghost Voltage, Ohms Law, Series and Parallel circuits, CKP, CMP, B+ and B-. Despite this alphabet soup I still can't figure out what my problem is.

I suspect that it may have something to do with temperature variance and some voltage " leakage" or inductance from one circuit to another or maybe the computer doesn't see any "information" and inserts something in place " as an expected " value. The VSS being 0 should tell the computer that the vehicle is standing still. The throttle is mechanical , so how is % and voltage changing?

NO DTC'S , intermittent and variable readings when it occurs is way above my pay grade.

So, if any one has an idea what is causing this, please let me know.

Thank you